A general imaging device such as a digital still camera has an autofocus unit (hereinafter, AF unit) which automatically focuses on a subject. As an example of an autofocus control method for use in the AF unit, a hill-climbing auto-focus method is known (for example, refer to JP S39-5265A). In this hill-climbing auto-focus method, an integral value of a luminance difference of adjacent pixels is obtained from picture signals output from an imaging element, and this integral value is used as an AF evaluation value showing a focus level. The contoured part of the subject in picture signals is clear when a subject is in a focus state, so that the luminance difference of the adjacent pixels in the picture signals is increased. Namely, the AF evaluation value is increased in a focus state. On the other hand, the contoured part of the subject is not clear when the subject is in a non-focus state, so that the luminance difference of the adjacent pixels in the picture signals is reduced. Namely, the AF evaluation value is reduced in a non-focus state.
In the AF unit, while moving a focus lens, an AF evaluation value is calculated by obtaining a picture signal at a predetermined timing or constant time intervals in a lens position at that time, the maximum value of the AF evaluation values (the peak position of the AF evaluation values) is specified, the focus lens is moved to the lens position where the AF evaluation value is the maximum value, and then the focus lens is stopped. Accordingly, the AF unit is configured to automatically focus on the subject. As described above, the hill-climbing auto-focus method focuses on a subject by detecting a peak position of the AF evaluation values.
In a conventional general hill-climbing auto-focus method, a lens position which can obtain the maximum AF evaluation value is specified by moving the lens over a predetermined entire movement range of the focus lens, and the focus lens is moved to the specified position. More specifically, an AF operation start position is used as the central position of the predetermined movement range of the focus lens, and the focus lens is firstly moved to that position. After moving the focus lens from that position to a certain direction, for example, the direction of the nearest focus position, the focus lens is turned over and is moved to the direction of the infinity focus position. During this time, the lens position which can obtain the maximum AF evaluation value is specified. However, according to such an AF control method, it takes a long time for the AF operation, and a photo opportunity may be missed.
In order to address such a problem, recently, an AF control method which improves a hill-climbing auto-focus method, and operates with high accuracy and at high speed has been proposed in JP3851027B, for example. The AF control method described in JP3851027B is a method which can increase an AF operation speed and can smoothly focus on a subject by using both of a first mode which samples AF evaluation values at minute intervals and a second mode which samples AF evaluation values at rough intervals until a focus lens comes closer to the focus position, and samples AF evaluation values at minute intervals near the focus position. However, in the AF control method described in JP3851027B, since the two AF controls such as the AF control at rough intervals and the AF control at minute intervals are performed, the increase in the speed of the focus operation is insufficient. For this reason, an increase in the speed of the focus operation is still desired.
Consequently, a focus adjuster described in JP2008-058559A is proposed. This focus adjuster is configured to store, after once focusing, when readjusting a focal point, an image in the last focus position, compare this stored image to a present image, and narrow a focus search range if the agreement is within a predetermined range. However, in the focus adjuster described in JP2008-058559A, it is premised that a once focused image is stored, and it does not follow a change in a subject over time. For this reason, if the subject is changed over time, the feature described in JP2008-058559A is more likely to be unused, so that the virtual effects can not be expected.
Additionally, all of the conventional autofocus adjusters have a problem in that a heavy lens has low stop accuracy in a focus position, has lack of movement by a driving pulse and requires time for movement or sometimes does not move. In particular, if a driving mechanism portion of a lens driving system has backlash, or a focus lens has a wide driving range such as a lens which can perform macro photography, the above problem is apparent.